Since its introduction in the 1970s, computed tomography (CT) has become an important tool in medical imaging to supplement X-rays and medical ultrasonography. It has more recently been used for preventive medicine or screening for disease, for example, CT colonography for patients with a high risk of colon cancer, or full-motion heart scans for patients with high risk of heart disease. A number of institutions offer full-body scans for the general population. However, this is a controversial practice, given its lack of proven benefit, its cost, the radiation exposure to the patient, and the risk of finding ‘incidental’ abnormalities that may trigger additional investigations.
CT scanning of the head is typically used to detect infarction, tumors, calcifications, hemorrhage and bone trauma. Of the above, hypodense (dark) structures indicate infarction and tumors, hyperdense (bright) structures indicate calcifications and hemorrhage, and bone trauma can be seen as disjunction in bone windows.
CT can be used for detecting both acute and chronic changes in the lung parenchyma, that is, the internals of the lungs. It is particularly relevant here because normal two dimensional x-rays do not show such defects. A variety of different techniques are used depending on the suspected abnormality. For evaluation of chronic interstitial processes (emphysema, fibrosis, and so forth), thin sections with high spatial frequency reconstructions are used—often scans are performed both in inspiration and expiration. This special technique is called High Resolution CT (HRCT). HRCT is normally done within thin slices with skipped areas between the thin slices. Therefore it produces a sampling of the lung and not continuous images. Continuous images are provided in a standard CT of the chest.
CT angiography (CTA) of the chest is also becoming the primary method for detecting pulmonary embolism (PE) and aortic dissection, and requires accurately timed rapid injections of contrast (Bolus Tracking) and high-speed helical scanners. CT is the standard method of evaluating abnormalities seen on chest X-ray and of following findings of uncertain acute significance. Cardiac CTA is now being used to diagnose coronary artery disease.
CT pulmonary angiogram (CTPA) is a medical diagnostic test used to diagnose pulmonary embolism (PE). It employs computed tomography to obtain an image of the pulmonary arteries. MDCT (multi detector CT) scanners give the optimum resolution and image quality for this test. Images are usually taken on a 0.625 mm slice thickness, although 2 mm is sufficient. 50-100 ml/s of contrast is given to the patient at the rate of 4 ml/s. The tracker/locator is placed at the level of the pulmonary arteries, which sit roughly at the level of the carina. Images are acquired with the maximum intensity of radio-opaque contrast in the pulmonary arteries. This is done using bolus tracking. CT machines are now so sophisticated that the test can be done with a patient visit of 5 minutes with an approximate scan time of only 5 seconds or less.
With the advent of subsecond rotation combined with multi-slice CT (up to 320-slices), high resolution and high speed can be obtained at the same time, allowing excellent imaging of the coronary arteries (cardiac CT angiography). Images with an even higher temporal resolution can be formed using retrospective ECG gating. In this technique, each portion of the heart is imaged more than once while an ECG trace is recorded. The ECG is then used to correlate the CT data with their corresponding phases of cardiac contraction. Once this correlation is complete, all data that were recorded while the heart was in motion (systole) can be ignored and images can be made from the remaining data that happened to be acquired while the heart was at rest (diastole). In this way, individual frames in a cardiac CT investigation have a better temporal resolution than the shortest tube rotation time.
Because the heart is effectively imaged more than once (as described above), cardiac CT angiography results in a relatively high radiation exposure, around 12 milliSievert (“mSv”). Currently, newer acquisition protocols have been developed drastically reducing the X-rays radiation exposure, down to 1 mSv (cfr. Pavone, Fioranelli, Dowe: Computed Tomography on Coronary Arteries, Springer 2009). For the sake of comparison, a chest X-ray carries a dose of approximately 0.02 to 0.2 mSv and natural background radiation exposure is around 0.01 mSv/day. Thus, cardiac CTA is equivalent to approximately 100-600 chest X-rays or over 3 years worth of natural background radiation.
Dual Source CT scanners, introduced in 2005, allow higher temporal resolution by acquiring a full CT slice in only half a rotation, thus reducing motion blurring at high heart rates and potentially allowing for shorter breath-hold time. This is particularly useful for ill patients who have difficulty holding their breath or who are unable to take heart-rate lowering medication.
The speed advantages of 64-slice MSCT have rapidly established it as the minimum standard for newly installed CT scanners intended for cardiac scanning. Manufacturers have developed 320-slice and true ‘volumetric’ scanners, primarily for their improved cardiac scanning performance.
The latest MSCT scanners acquire images only at 70-80% of the R-R interval (late diastole). This prospective gating can reduce effective dose from 10-15 mSv to as little as 1.2 mSv in follow-up patients acquiring at 75% of the R-R interval. Effective doses with well trained staff doing coronary imaging can average less than the doses for conventional coronary angiography.
CT is a sensitive method for diagnosis of abdominal diseases. It is used frequently to determine stage of cancer and to follow progress. It is also a useful test to investigate acute abdominal pain (especially of the lower quadrants, whereas ultrasound is the preferred first line investigation for right upper quadrant pain). Renal stones, appendicitis, pancreatitis, diverticulitis, abdominal aortic aneurysm, and bowel obstruction are conditions that are readily diagnosed and assessed with CT. CT is also the first line for detecting solid organ injury after trauma.
Multidetector CT (MDCT) can clearly delineate anatomic structures in the abdomen, which is critical in the diagnosis of internal diaphragmatic and other nonpalpable or unsuspected hernias. MDCT also offers clear detail of the abdominal wall allowing wall hernias to be identified accurately. CT is often used to image complex fractures, especially ones around joints, because of its ability to reconstruct the area of interest in multiple planes. Fractures, ligamentous injuries and dislocations can easily be recognized with a 0.2 mm resolution.
There are several advantages that CT has over traditional 2D medical radiography. First, CT completely eliminates the superimposition of images of structures outside the area of interest. Second, because of the inherent high-contrast resolution of CT, differences between tissues that differ in physical density by less than 1% can be distinguished. Finally, data from a single CT imaging procedure consisting of either multiple contiguous or one helical scan can be viewed as images in the axial, coronal, or sagittal planes, depending on the diagnostic task. This is referred to as multiplanar reformatted imaging.
CT is regarded as a moderate to high radiation diagnostic technique. While technical advances have improved radiation efficiency, there has been simultaneous pressure to obtain higher-resolution imaging and use more complex scan techniques, both of which require higher doses of radiation. The improved resolution of CT has permitted the development of new investigations, which may have advantages; compared to conventional angiography for example, CT angiography avoids the invasive insertion of an arterial catheter and guide wire; CT colonography (also known as virtual colonoscopy or VC for short) may be as useful as a barium enema for detection of tumors, but may use a lower radiation dose. VC is increasingly being used as a diagnostic test for bowel cancer and can negate the need for a colonoscopy.
The greatly increased availability of CT, together with its value for an increasing number of conditions, has been responsible for a large rise in popularity. So large has been this rise that, in the most recent comprehensive survey in the United Kingdom, CT scans constituted 7% of all radiologic examinations.
The radiation dose for a particular study depends on multiple factors: volume scanned, patient build, number and type of scan sequences, and desired resolution and image quality. Additionally, two helical CT scanning parameters—tube current and pitch—can be adjusted easily and have a profound effect on radiation dose.
Typical effectiveExaminationdose (mSv)(millirem)Chest X-ray0.1 10Head CT1.5150Screening mammography3300Abdomen CT5.3530Chest CT5.8580CT colonography (virtual colonoscopy)3.6-8.8360-880Chest, abdomen and pelvis CT9.9990Cardiac CT angiogram6.7-13  670-1300Barium enema151500 Neonatal abdominal CT202000 
From the foregoing, it will be appreciated that CT scanning is growing in popularity and is being used to image human tissues literally all over the body. The inventors view the recent reports of energetic X-rays from CT scans as being the “tip of an iceberg.” That is fast CT scans are increasingly being used for thoracic imaging where cardiac pacemakers are typically implanted. However, AIMDs are increasingly being implanted all over the human body as well. Accordingly, the chance for energetic X-ray from either diagnostic or therapeutic devices being imaged directly onto an AIMD is increasing and will continue to increase in the future.
X-ray slice data is generated using an X-ray source that rotates around the object; X-ray sensors are positioned on the opposite side of the circle of rotation from the X-ray source. The earliest sensors were scintillation detectors, with photomultiplier tubes excited by (typically) cesium iodide crystals. Cesium iodide was replaced during the 1980s by ion chambers containing high pressure Xenon gas. These systems were in turn replaced by scintillation systems based on photo diodes instead of photomultipliers and modern scintillation materials with more desirable characteristics. Many data scans are progressively taken as the object is gradually passed through the gantry.
Newer machines with faster computer systems and newer software strategies can process not only individual cross-sections but continuously changing cross-sections as the gantry, with the object to be imaged, is slowly and smoothly slid through the X-ray circle. These are called helical or spiral CT machines. Their computer systems integrate the data of the moving individual slices to generate three-dimensional volumetric information (3D-CT scan), in turn viewable from multiple different perspectives on attached CT workstation monitors. This type of data acquisition requires enormous processing power, as the data are arriving in a continuous stream and must be processed in real-time.
In conventional CT machines, an X-ray tube and detector are physically rotated behind a circular shroud (FIGS. 1-4); in electron beam tomography (EBT) the tube is far larger and higher power to support the high temporal resolution. The electron beam is deflected in a hollow funnel-shaped vacuum chamber. X-rays are generated when the beam hits the stationary target. The detector is also stationary. This arrangement can result in very fast scans, but is extremely expensive.
Once the scan data has been acquired, the data must be processed using a form of tomographic reconstruction, which produces a series of cross-sectional images. The most common technique in general use is filtered back projection, which is straight-forward to implement and can be computed rapidly. Mathematically, this method is based on the Radon transform. However, this is not the only technique available: the original EMI scanner solved the tomographic reconstruction problem by linear algebra, but this approach was limited by its high computational complexity, especially given the computer technology available at the time. More recently, manufacturers have developed iterative physical model-based expectation-maximization techniques. These techniques are advantageous because they use an internal model of the scanner's physical properties and of the physical laws of X-ray interactions. By contrast, earlier methods have assumed a perfect scanner and highly simplified physics, which leads to a number of artifacts and reduced resolution—the result is images with improved resolution, reduced noise and fewer artifacts, as well as the ability to greatly reduce the radiation dose in certain circumstances. The disadvantage is a very high computational requirement, which is at the limits of practicality for current scan protocols.
Pixels in an image obtained by CT scanning are displayed in terms of relative radiodensity. The pixel itself is displayed according to the mean attenuation of the tissue(s) that the pixel corresponds to on a scale from +3071 (most attenuating) to −1024 (least attenuating) on the Hounsfield scale. A pixel is a two dimensional image unit (picture element—pixel) based on the matrix size and the field of view. When the CT slice thickness is also factored in, the unit is known as a Voxel, which is a three dimensional unit. The phenomenon that one part of the detector cannot differentiate between different tissues is called the “Partial Volume Effect”. That means that a large amount of cartilage and a thin layer of compact bone can cause the same attenuation in a voxel as hyperdense cartilage alone. Water has an attenuation of 0 Hounsfield units (HU) while air is −1000 HU, cancellous bone is typically +400 HU, cranial bone can reach 2000 HU or more (os temporale) and can cause artifacts. The attenuation of metallic implants depends on the atomic number of the element used: titanium usually has an amount of +1000 HU, iron steel can completely extinguish the X-ray and is therefore responsible for well-known line-artifacts in computed tomograms. Artifacts are caused by abrupt transitions between low and high-density materials, which results in data values that exceed the dynamic range of the processing electronics.
Because contemporary CT scanners offer isotropic or near isotropic, resolution, display of images does not need to be restricted to the conventional axial images. Instead, it is possible for a software program to build a volume by “stacking” the individual slices one on top of the other. The program may then display the volume in an alternative manner.
Multiplanar reconstruction (MPR) is the simplest method of reconstruction. A volume is built by stacking the axial slices. The software then cuts slices through the volume in a different plane (usually orthogonal). Optionally, a special projection method, such as maximum-intensity projection (MIP) or minimum-intensity projection (mIP), can be used to build the reconstructed slices.
MPR is frequently used for examining the spine. Axial images through the spine will only show one vertebral body at a time and cannot reliably show the intervertebral discs. By reformatting the volume, it becomes much easier to visualize the position of one vertebral body in relation to the others.
Modern software allows reconstruction in non-orthogonal (oblique) planes so that the optimal plane can be chosen to display an anatomical structure. This may be particularly useful for visualizing the structure of the bronchi as these do not lie orthogonal to the direction of the scan.
For vascular imaging, curved-plane reconstruction can be performed. This allows bends in a vessel to be “straightened” so that the entire length can be visualized on one image, or a short series of images. Once a vessel has been “straightened” in this way, quantitative measurements of length and cross sectional area can be made, so that surgery or interventional treatment can be planned.
MPR reconstructions enhance areas of high radiodensity, and so are useful for angiographic studies. MPR reconstructions tend to enhance air spaces so are useful for assessing lung structure.
Surface rendering: a threshold value of radiodensity is set by the operator (e.g. a level that corresponds to bone). From this, a three-dimensional model can be constructed using edge detection image processing algorithms and displayed on screen. Multiple models can be constructed from various different thresholds, allowing different colors to represent each anatomical component such as bone, muscle, and cartilage. However, the interior structure of each element is not visible in this mode of operation.
Volume rendering: Surface rendering is limited in that it will only display surfaces which meet a threshold density, and will only display the surface that is closest to the imaginary viewer. In volume rendering, transparency and colors are used to allow a better representation of the volume to be shown in a single image—e.g. the bones of the pelvis could be displayed as semi-transparent, so that even at an oblique angle, one part of the image does not conceal another.
As previously mentioned, there have been a number of recent reports of X-ray radiation causing interference in implantable cardiac pacemakers.    Reference 1 X-RAY RADIATION CAUSES ELECTROMAGNETIC INTERFERENCE IN IMPLANTABLE CARDIAC PACEMAKERS, Minour Hirose, Ph.D., Keiichi Tachikawa, C.C.E., Masanori Ozaki, R.T., Naoki Umezawa, R.T., Toshihiro Shinbo, C.C.E., Kenichi Kokubo, Ph.D., and Hirosuke Kobayashi, M.D., Ph.D., from the Department of Medical Engineering and Technology, School of Allied Health Services, Kitasato University, Kanagawa, Japan; Department of Clinical Engineering, Jichi Medical University Hospital, Tochigi, Japan; and Department of Radiological Technology, School of Allied Health Sciences, Kitasato University, Kanagawa, Japan, PACE, Vol. 33, October 2010, No. 1174. The conclusion was that “X-ray radiation caused interference in some implantable cardiac pacemakers, probably because the CMOS (microelectronic) component was irradiated. The occurence of EMI depended on the pacemaker model, sensing threshold of the pacemaker, and X-ray radiation conditions, PACE 2010; 1174-1181.”    Reference 2 “RADIATION THERAPY IN ONCOLOGY PATIENTS WHO HAVE A PACEMAKER OR IMPLANTABLE CARDIOVERTER-DEFIBRILLATOR,” Bart Frizzell, MD, Department of Radiation Oncology, Wake Forest University, Winston-Salem, N.C., and Department of Radiation Oncology, High Point Regional Cancer Center, High Point, N.C., Technology, Community Oncology, October 2009, Volume 6/Number 10.    Reference 3 On Jul. 14, 2008, the FDA released a warning. FDA PRELIMINARY PUBLIC HEALTH NOTIFICATION: POSSIBLE MALFUNCTION OF ELECTRONIC MEDICAL DEVICES CAUSED BY COMPUTED TOMOGRAPHY (CT) SCANNING, Date Jul. 14, 2008. This FDA public health notification is available on the Internet at http://www.fda.gov/cdrh/safety.html. Questions about the FDA notification can be answered at the Office of Surveillance and Biometrics (HFZ-520), 1350 Piccard Drive, Rockville, Md., 20850.
Also cited in the FDA article are references 4, 5 and 6 listed below:    Reference 4 “DOES HIGH-POWER COMPUTED TOMOGRAPHY SCANNING EQUIPMENT AFFECT THE OPERATION OF PACEMAKERS?,” Yamaji, S., et al., Circulation Journal 70:190-197 (2006).    Reference 5 “EFFECTS OF CT IRRADIATION ON IMPLANTABLE CARDIAC RHYTHM MANAGEMENT DEVICES,” McCollough, C., et al., Radiology 243 (3):766-774 (2007).    Reference 6 “HAZARD REPORT—CT SCANS CAN AFFECT THE OPERATION OF IMPLANTED ELECTRONIC DEVICES,” ECRI Institute Problem Reporting System, Health Devices 36 (4):136-138 (2007).
Additional references include:    Reference 7 “DOES HIGH-POWER COMPUTED TOMOGRAPHY SCANNING EQUIPMENT AFFECT THE OPERATION OF PACEMAKERS?”, Satoshi Yamaji, MD; Shinobu Imai, MD; Funio Saito, MD; Hiroshi Yagi, MD; Toshio Kushiro, MD; Takahisa Uchiyama, MD, Circulation Journal, Volume 70, February 2006. “Conclusions: Malfunctions of the pacemaker during CT may be caused by diagnostic radiant rays and although they are transient, the possibility of lethal arrhythmia cannot be ignored. (Circ J 2006; 70:190-197).”    Reference 8 “EFFECTS OF CT IRRADIATION ON IMPLANTABLE CARDIAC RHYTHM MANAGEMENT DEVICES,” Cynthia H. McCollough, PhD; Jie Zhang, PhD; Andrew N. Primak, PhD; Wesley J. Clement, BSEE; John R. Buysman, PhD, from the Department of Radiology, Mayo Clinic College of Medicine, 200 First St. SW, Rochester, Minn., 55905 (C.H.M., J.Z., A.N.P.); and Cardiac Rhythm Disease Management, Medtronic, Minneapolis, Minn. (W.J.C., J.R.B.). Received Jun. 8, 2006; revision requested August 10; revision received November 21; final version accepted December 6. Address correspondence to C.H.M. (email: mccollough.cynthis@mayo.edu), Radiology: Volume 243: Number 3—June 2007. In this article, 13 pacemaker models and 8 ICD models were exposed to ionizing radiation from both spiral and dynamic CT at maximum typical dose levels. Two different CT systems were chosen to represent current state-of-the-art technology: a 16-channel system (LightSpeed 16; GE Healthcare, Waukesha, Wis. and a 64-channel system (Sensation 64; Siemens Medical Solutions, Forchheim, Germany). The study revealed that ionizing radiation from CT examinations can indeed influence implantable device operation. The effects included over-sensing (inhibition), tracking, and safety pacing, and partial electrical reset (PER). The effects observed were associated with the direct radiation of the AIMD housing (electronics module). No device in this particular study was permanently damaged.
There have been a number of recent reports and publications that high-dose-rate computed tomography (CT scanners) can interfere with cardiac pacemakers or ICDs. X-ray computed tomography or CT is a medical imaging modality employing computer processing to reconstruct two- or three-dimensional images from the transmission of X-rays through the body. Digital geometry processing is used to generate a three-dimensional image of the inside of an object from a large series of two-dimensional X-ray images taken around a single axis of rotation.
CT produces a volume of data which can be manipulated, through a process known as “windowing,” in order to demonstrate various bodily structures based on their ability to block the X-ray beam. Although historically the images generated were in the axial or transverse plane, orthogonal to the long axis of the body, modern scanners allow this volume of data to be reformatted in various planes or even as volumetric (3D) representations of structures.
Usage of CT has increased dramatically over the last two decades. An estimated 72 million scans were performed in the United States in 2007.
Recently, CT scanners have evolved to operate at increasingly higher dose rates. Experience by NASA in space radiation environments has demonstrated that microcircuits and/or electronic chips can be affected by various forms of radiation including high-energy photons (e.g. X-rays and Gamma rays), and particles such as energetic protons. NASA has solved these problems by bulky radiation shields, such as shields made of light weight material such as aluminum. In addition to or in lieu of shielding, NASA engineers and their subcontractors have also engineered radiation hardened (rad-hard) chip sets. Rad-hard chipsets can be realized either through circuit design techniques (rad-hard by design) or through materials engineering. Rad-hard by design consists of approaches such as triple modular redundancy (TMR). For example, in a microelectronic chip there could be three redundant parallel gates. The voting process entails the following logic: if gate 1 was giving an indication of a one and gates 2 and 3 were giving an indication of zero, then a zero would be voted on/selected. Probability and statistics have indicated that the likelihood of two or more gates being affected by a radiation bit flip at the same time is extremely unlikely.
Unfortunately, when considering active implantable medical devices, all of these prior art methods have several problems associated with them. In order for aluminum or other low-cost light-weight materials to be used as an effective shield, they need to be relatively thick because of their low densities and low atomic numbers. These thicknesses are far too thick to be of any practical benefit for use in an active implantable medical device. In addition, AIMDs such as cardiac pacemakers have evolved to be very small in size. Using a redundant rad-hard gate voting technique would more than double (potentially triple) the size of the chip set. There is simply not enough room in a modern cardiac pacemaker to accomplish this. To fully understand the issues, one needs to also understand the evolution of X-ray sources as used in medical diagnostic procedures such as CT scans. Not only have pacemakers been evolving to be much smaller, these X-ray sources have been evolving so that they are now operating at much higher dose rates and energies than ever before.
Accordingly, there is a need for radiation shielding for active implantable medical devices that meets the requirements of being lightweight, volumetrically efficient, and of reasonable cost. This type of shielding is not satisfied by traditional approaches such as the lead vest worn by X-ray technicians. Such shields are way too bulky and heavy.